This invention relates to electrolytic cells for the production of aluminum. More particularly, it relates to a novel and improved composite strata which is disposed between the carbonaceous lining and the refractory insulating layer of the cell; said strata preventing distortion and deterioration of the lining, thereby extending the lining life of the cell.
The production of aluminum by the electrolytic reduction of alumina dissolved in a molten salt electrolyte, such as cryolite, is an old and well-known process commonly termed the "Hall-Heroult process". The alumina which is dissolved in the molten or fused electrolyte breaks down into its components, the oxygen being liberated at the anode and metallic aluminum being deposited in a pool or body of molten metal which forms at the bottom of the electrolytic cell. The body of molten aluminum which is formed in the bottom portion of the cell in effect constitutes the cathode of the cell.
There are two types of electrolytic cells for the production of aluminum, namely, the "prebake" cell and the "Soderberg" cell. With either cell the reduction process involves precisely the same chemical reaction. The principal difference between the two cells is one of structure. In the prebake cell the carbon anodes are prebaked before being installed in the cell, whereas in the Soderberg cell, or sometimes referred to as the continuous anode cell, the anode is baked in situ, that is, it is baked during the operation of the cell, thereby utilizing part of the heat generated by the reduction process. The fused electrolyte or bath employed in the Hall-Heroult process consists essentially of cryolite which is a double salt of sodium fluoride and aluminum fluoride having the formula Na.sub.3 AlF.sub.6, or, expressed in another manner, 3NaF.AlF.sub.3. Cryolite has a melting point of about 1000.degree. C. Other compounds, including aluminum fluoride up to 10% in excess of the stoichiometric amount of aluminum fluoride in cryolite, 5 to 15% of calcium fluoride, and sometimes several percent of LiF, MgF.sub.2 and/or NaCl, may be added to the electrolyte to reduce its liquidus temperature and modify or control such other properties as electrical conductivity, viscosity and surface tension. Alumina concentration is normally maintained between about 2 and 10% by weight. As aluminum metal is produced, the concentration of the alumina decreases and must be periodically replenished.
The conventional aluminum reduction cell is generally comprised of a steel shell, a current-carrying carbonaceous lining disposed therein and one or more carbon anodes disposed within a cavity defined by the carbonaceous lining. The carbonaceous cathode lining may be a monolithic lining which is tamped into place and baked in during the operation of the cell or it may be composed of carbonaceous blocks which have been baked prior to installation in the cell. Embedded in the cathode lining are a plurality of collector bars. Normally, insulating material such as granular alumina or refractory brick is disposed between the steel shell and the carbonaceous lining to conserve the heat generated during the electrolytic process. In many instances the insulating layer is provided only on the bottom portion of the steel shell.
During the service life of the electrolytic cell the carbon linings are subjected to severe chemical and temperature conditions which are deleterious to the carbon lining and consequently the cells have uncertain service lives which may vary from a few days to thousands of days. However, essentially all early cell failures, other than those which stem directly from inadvertently faulty workmanship or other mishaps in a cell's construction, are thought to result because electrolyte penetrates and freezes within the pores and capillary passageways of the carbonaceous lining where it then reacts with elemental sodium to produce reaction products having substantially greater volumes than the original reactants. Where this sodium reaction occurs with electrolyte that is still liquid within the carbon pores, the increased volume of the reaction products can be harmlessly accommodated by an upward displacement of a portion of the overlying liquid within the carbon's capillaries. But where this reaction occurs with electrolyte that has already frozen and been solidly confined within the carbon pores, the increased volume of the reaction products causes a local expansive stress that cracks and comminutes the carbon immediately neighboring the reaction sites. A source of sodium vapor for these reactions is available at all interior surfaces (i.e., pore walls), as well as the exterior surfaces, of the carbon lining because of the well-known sodium intercalation reaction with incompletely graphitized carbon, as described, for example, by E. W. Deming, Trans. AIME, vol. 227, December 1963, pp 1328-1334. The principal expansion is thought to result from one or both of the reactions EQU Na.sub.3 AlF.sub.6 +3Na=Al+6NaF
and EQU 4Na.sub.3 AlF.sub.6 +12Na+3C=Al.sub.4 C.sub.3 +24NaF
although other sodium reduction reactions, with CaF.sub.2 for example, may also be involved. As a general principle, therefore, it is desirable that the electrolyte should neither freeze nor solidify within the carbon.
For energy efficiency, Hall-Heroult cells are commonly designed with enough bottom insulation so that the isotherm for solidification of the electrolyte lies principally in the insulation beneath the carbon, at least initially. During cell operation, however, the insulation is exposed to sodium vapor, fluoride fumes, and infiltration by the molten electrolyte itself, all of which tend to damage the insulation and reduce its insulating value so that the solidification isotherm eventually retreats into the carbon.
Accordingly, it would appear that a barrier of some sort disposed within the electrolytic cell would be required to shield the insulation and to protect it from deterioration by the penetration of molten electrolyte, the penetrations of sodium vapor and the fluoride fumes of the carbon lining and the avoidance of freeze back of the electrolyte or bath into the carbon. In the prior art many barriers have been disclosed and recommended for prolonging the life of carbon linings. For example, overlapping sheets of steel plates disposed between the insulation and the carbon lining have been proposed and have been used in linings of electrolytic cells for aluminum for many years. Also, it has been suggested that GRAFOIL.RTM., a registered trademark of High Temperature Materials, Inc., be used as a covering layer over the overlapped steel plates. However, the latter material is fragile and expensive, and neither are efficacious in stopping the advance of sodium and fluoride vapors.
The problem of insulation deterioration by penetration of molten electrolyte, sodium vapor and fluoride fumes into the carbon lining of the cell has been recognized in prior patents in regard to electrolytic cells for the production of aluminum. U.S. Pat. No. 3,457,149 to Arthur F. Johnson concerning a method of forming cathode linings proposes a process for filling the pores and fissures of the linings by vacuum-assisted impregnation of the pores and fissures with low melting point halides, such as, calcium chloride or magnesium chloride or sodium chloride to which has been added aluminum chloride or mixtures of fluorides. The process in U.S. Pat. No. 3,457,149 has the serious fault that the carbon ultimately becomes hot enough to melt the low melting point pore filling mixtures, after which they simply dissolve in the bath and their desired sealing effect is lost.
U.S. Pat. Nos. 3,434,957 and 3,649,480, both to Arthur F. Johnson, propose the use of a refractory layer disposed in the lining of the cell such as a refractory coated paper or a paint of aluminum silicate or sodium silicate. Johnson proposes disposing the thin layers between the insulation and carbon lining layers, as well as using it as a paint on the inside of the steel shell of the cell to inhibit tapouts of the molten aluminum.
U.S. Pat. No. 3,514,520 to Bacchiega et al proposes the forming of a barrier between layers of the lining material of an electrolytic reduction furnace for aluminum of powdered or granulated silicon carbide in an incoherent state. According to the patent, this silicon carbide layer constitutes a barrier unsurmountable by molten aluminum.
U.S. Pat. No. 4,033,836 to G. Thomas Holmes proposes the disposition of a layer of aluminum fluoride intermediate the metal shell and the layer of carbonaceous material of the lining of an aluminum electrolytic reduction furnace. This supposedly prevents corrosion of the metal shell by the sodium.
U.S. Pat. No. 3,723,286 to Leland F. Hunt et al proposes the incorporation of a layer of salt such as chloride and fluoride salts of sodium, lithium, calcium and magnesium between the carbon lining and the insulating lining of an electrolytic cell for aluminum to prevent distortion of the carbon lining.
In the production of aluminum metal by the aluminum chloride process, there is a problem with the corrosivity of the chloride bath and its ability to penetrate the refractory linings and attack the steel shell, particularly when the cell is operated at elevated temperatures, e.g., above the melting point of aluminum. At these temperatures, there is a rapid seepage through the cell walls of the electrolytic bath components resulting in a rapid attack of the cell walls. The electrolytes used in the aluminum chloride process, usually composed of aluminum chloride with other chlorides, such as sodium chloride, potassium chloride and lithium chloride, that is, the alkali metal chlorides, are considerably different from the cryolitic electrolytes employed in the Hall-Heroult process; consequently, the types of corrosion and deterioration in the two systems are of substantial difference. In the aluminum chloride process the cell is closed because of the generation of chlorine gas which is highly corrosive to the steel parts of the cell. There are a number of patents which disclose schemes for protecting the steel shell from the detrimental corrosion of the chlorides. U.S. Pat. No. 3,773,643 and 3,779,699, both to Russell et al, propose the interposition of a glass barrier between the steel shell of the cell and the insulation layer of a suitable material, such as refractory bricks. These patents disclose the use of a plurality of glass layers for the barrier. The blass barrier is effectively impervious to penetration by the molten chloride seeping laterally into the sidewalls of the cell.